The present invention relates to a reading method and reading apparatus of magneto-optical recording medium, and more particularly to a reading method and reading apparatus of magneto-optical recording medium capable of reading at magnetically induced super resolution (MSR).
The magneto-optical disk and its recording and reading apparatus are rapidly expanding in the market owing to the large capacity, reloadability and high reliability of the devices. With such apparatus, good recording reading of picture, image information, and computer code information are realized. It is desired to increase further the memory capacity of such magneto-optical disk. To increase the recording density, it is necessary to form more record marks on the medium, and it is hence needed to set the recording mark length shorter than the spot diameter of the laser beam, and fill in recording mark gaps. The recording density of recording marks in magneto-optical recording and reading is limited by the spot diameter of the light beam for irradiating the disk. It is relatively easy to form a fine recording mark having a period shorter than the spot diameter, but there was a limit to the length of the recording mark that can be reproduced, due to restriction of the wavelength .lambda. the laser beam to irradiate, and the numerical aperture NA of the objective lens when reproducing a fine recording mark.
Accordingly, by making use of the temperature distribution of the medium produced in the spot of the light beam, and reading out the recording marks (bits) from part of a region in the spot, the MSR reading method and MSR medium in which the spot diameter is reduced are proposed in Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93058 (1991), etc. The former is a magneto-optical reading method of emitting the light beam while applying a reading magnetic field to a magneto-optical disk of multi-layer structure laminating a reading layer, a switch layer, and a recording layer on a substrate. In reading, a temperature distribution is formed in the beam spot due to rotation of the magneto-optical disk, and a high temperature region and a low temperature region are formed. In the low temperature region, by the exchange coupling force between the recording layer and reading layer through the switch layer, bits of the recording layer are transferred to the reading layer and read out. In the high temperature region, since the exchange coupling force between the recording layer and reading layer is cut off, the magnetization of the reading layer is aligned in the direction of the reading magnetic field, and the bits in the recording layer are masked. As a result, the bits are reproduced only from the low temperature region in the spot (FAD system), and the reading resolution is enhanced that same as when the beam spot is substantially reduced.
FIG. 1 is a diagram showing the film composition of the MSR medium proposed in the latter Japanese Patent Application Laid-Open No. 3-93058 (1991), and the state of magnetization when reproducing. This is a method of magneto-optical reading for emitting a light beam while applying an initial magnetic field and a reading magnetic field Hr to a magneto-optical disk of multi-layer structure laminating a reading layer 41, a reading auxiliary layer 42, an intermediate layer 43, and a recording layer 44 on a substrate (not shown). When reproducing, by rotation of the magneto-optical disk, a temperature distribution occurs in the beam spot S, and a high temperature region, a low temperature region, and an intermediate temperature region between them are formed. In the low temperature region, since the intermediate layer 43 has an in-plane magnetization characteristic, the exchange coupling force between the recording layer 44 and the reading layer 41 is cut off. The magnetization of the reading layer 41 is aligned in the direction of the initializing magnetic field, and the bits of the recording layer 44 are masked (front mask). In the high temperature region, the reading auxiliary layer 42 exceeds the Curie temperature, and the exchange coupling force between the recording layer 44 and reproducing layer 41 is cut off. The magnetization of the reading layer 41 is aligned in the direction of the reading magnetic field, so that the bits of the recording layer 44 are masked (rear mask).
In the intermediate temperature region, the intermediate layer 43 has a perpendicular magnetization characteristic, and the bits of the recording layer 44 are transferred to the reading layer 41 through the intermediate layer 43 and the reading auxiliary layer 42, and read out (the opening). As a result, the bit is reproduced only from the intermediate temperature region in the beam spot S. The result and is substantially the same as when the beam spot is reduced, and reading resolution is enhanced (RAD double mask system).
In the MSR reading method proposed in Japanese Patent Application Laid-Open No. 3-93058 (1991), it was necessary to align the magnetization of the reading layer 41 and the reading auxiliary layer 42 in the same direction by applying an initializing magnetic field of several kOe to the magneto-optical disk by an initializing magnet 45. This is because the coercive force of the reading layer 41 and the reading auxiliary layer 42 is larger than the exchange coupling force from the recording layer 44 through the intermediate layer 43 in the low temperature region. In the transfer region which is the intermediate temperature region, along with elevation of temperature of the magneto-optical disk, the magnitude relation is reversed, so that an exchange coupling force acts.
FIG. 2 is a block diagram showing a construction of a conventional magneto-optical reading apparatus. In the diagram, reference numeral 16 is a magneto-optical disk, which is an MSR medium of the RAD double mask system as mentioned above. In the magneto-optical disk 16, a recording mark is formed by using a laser pulse magnetic field modulation recording system. The laser pulse magnetic field modulation recording system is a kind of magnetic field modulation recording, and is a recording method of emitting laser beam generating pulses, while applying a magnetic field modulated on the basis of the information to be recorded. By these recording methods, finer recording marks can be formed.
An optical head 2 is disposed at one side of such magneto-optical disk 16. The optical head 2 has a laser light source, an optical system for guiding the laser beam emitted from the laser light source, and converting means for converting the magneto-optical signal into an electric signal. On other side of the magneto-optical disk 16, a magnetic coil 3 is disposed, and a reading magnetic field in predetermined direction is applied to the magneto-optical disk 16 by the signal given from a magnetic coil driving circuit 4. The laser beam emitted from the optical head 2 irradiates the magneto-optical disk 16, and its reflected light is condensed, and a magneto-optical signal is detected, and is converted into a reading signal which is an electric signal.
The reading signal produced from the optical head 2 is put into an amplifier 5 and amplified, and is put out into an AGC (automatic gain control) circuit 6. The gain of the signal put in the AGC circuit 6 is adjusted, and is delivered into an equalizer 7. The waveform is equalized, and the waveform equalized signal is put out into an LPF (low pass filter) 8, and high frequency noise is removed. The signal from the LPF 8 is sent out into a binary circuit 9 to be transformed into a binary signal, and it is further put out into a data discriminator 10 and a PLL (phase locked loop) 11. The binary signal entering the data discriminator 10 receives the signal issued from the PLL 11, and separate data and clock signals are put into a demodulator 12. The entered separate data is demodulated in the demodulator 12. In this way, the information recorded in the magneto-optical disk 16 is reproduced.
FIG. 3 and FIG. 4 are diagrams showing the recording mark formed in the magneto-optical disk 16, and the waveforms of reproduction signals obtained by applying reading magnetic fields of minus magnetic field and plus magnetic field, displaying the phase distributions of both the leading edge and trailing edge of the waveform of reproduction signal. In the recording mark, the bit of which magnetizing direction is the recording direction is indicated by hatching. The slope of the edge of the reproduction signal in such MSR reading is steep as compared with the case of ordinary reading in which all regions of the beam spot S are in the opening. In this case the jitter of the binary signal to the same noise power is small, because in MSR reading, the bit is read out only in part of the region close to the middle of the beam spot S, and the reproduction signal is obtained, corresponding to the light intensity near the middle of the Gaussian distribution of the beam spot S.
The inventors discovered that the magnet-optical disk 16 described above has reproductions characteristics which will now be described.
FIG. 3 and FIG. 4 show the effects of setting the direction of reading magnetic field in the reverse direction (minus magnetic field) to the recording direction and the same direction (plus magnetic direction). When the minus magnetic field is applied, the waveform of the reproduction signal is moderate in the inclination of the leading edge as compared with the inclination of the trailing edge, as shown in FIG. 3. When the reading magnetic field in the plus magnetic field is applied, as shown in FIG. 4, the inclination of the trailing edge is moderate as compared with the inclination of the leading edge. This is due to the difference in the forming ranges of the front mask and rear mask in the beam spot S. The inclination is steeper in the edge of the mask forming in the range close to the middle of the beam spot S.
Also shown FIG. 3 and FIG. 4 are the edge phase distributions. The edge phase distribution expresses the phase deviation due to noise contained in the reproduction signal, showing the magnitude of the jitter. When the minus magnetic field is applied, the phase distribution of the leading edge is wider than that of the trailing edge of the reproduction signal, and the jitter is larger. When the plus magnetic field is applied, the phase distribution is wider in the trailing edge than in the leading edge, and the jitter is larger.
FIG. 5 is a graph showing the jitter characteristic relative to the recording density of the magneto-optical disk. The ordinates denotes the jitter, and the abscissas represents the dimension of the recording mark. As shown in the graph, as the recording mark becomes smaller, that is, as the recording density becomes higher, the jitter becomes larger in both leading edge and trailing edge. Since the reproduction signal is determined by the average of the jitter of the leading edge and the trailing edge, as shown in FIG. 3 and FIG. 4, if the jitter of one edge is large, this jitter has a large effect on the quality of the reproduction signal. In particular, in a high recording density medium such as MSR medium, the reproduction signal asymmetrical in edge is extremely deteriorates to quality.